Thermosetting plastic material is a material that is in a viscous or plastic state when it is cooled and irreversibly solidifies or sets when heat and pressure are applied to it. This property is caused by the cross-linking reactions of the molecular constituants of the material which may include "curing" agents such as organic peroxides. Examples of such plastic material are glass-filled polyester or glass-filled epoxy. Once the plastic material hardens, the material cannot return to the plastic state. This is different from a thermoplastic material which sets at room temperature but which can return to the liquid or plastic state when heated.
A conventional way of making parts of thermosetting plastic having irregular shapes is from sheet molding compound (S.M.C.). Initially, a plastic sheet is formed which contains glass fibers. The sheet is then placed in the cavity portion of a two-part mold, and heat and pressure are applied to compress the sheet to form it into the shape of the mold cavity. The heat and pressure applied to the sheet hardens the sheet into the shape of the mold cavity defined by the molding surfaces of the mold parts. The mold parts are thereafter separated to remove the hardened plastic part from the mold.
Another way of making such plastic parts is to injection mold the thermosetting plastic material from a bulk molding compound (B.M.C.). The powdery molding compound is first mixed with a binder and transformed into a highly viscous mass. The highly viscous mass flows under pressure through a manifold, through one or more nozzles and into the mold cavity under pressure. The mass changes from its highly viscous state to a liquid state (i.e. its kick-over state) and pressure is applied thereto. In the liquid state the mass conforms to the size and shape of the cavity and crosslinking occurs as additional heat and pressure are applied until the mass becomes solid.
The number of nozzles is determined by the size and shape of the particular molded plastic part (i.e. the size and shape of the mold cavity). The manifold and the nozzles must be constantly cooled such as by cooling circulating water so that the thermosetting plastic material does not set within the manifold or within the nozzles.
Oftentimes, the cooling circulating water is inadvertently turned off when the thermosetting plastic material is in the manifold and/or the nozzles. When this happens, the temperature in the nozzles and the manifold rises due to the elevated temperature of the heated mold parts, thereby causing the plastic to set in the manifold and the nozzles. When this occurs, the only way that the hard thermosetting plastic material can be removed is by mechanically grinding the plastic material into powder by a special tool. This is a time consuming operation because frequently the manifold and nozzles are located in a hard to reach location and the inner passage or passages of the manifold are relatively inaccessible. It is also a costly operation since while the manifold and the nozzles are being cleaned the molding machine lies idle.